The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, or the like, the charging means of which for charging an image bearing means such as an electrophotographic photoconductive member, an electrostatically recordable dielectric member, or the like, is such a charging means that employs electrically conductive particles, and in which the electrically conductive particles are supplied from the developing means to the nip portion between the charging member and the image bearing member, by way of the image bearing member.
Conventionally, in an image forming apparatus, for example, an electrophotographic image forming apparatus or an electrostatic recording apparatus, an electrophotographic latent image is formed on an image bearing member such as an electrophotographic photoconductive member, an electrostatically recordable dielectric member, and the like. In order to form the electrophotographic image on the image bearing member, the image bearing member must be uniformly charged. As for a charging apparatus for uniformly charging the image bearing member, a corona type charging apparatus (which is not placed in contact with image bearing member) has been widely used. However, a corona type charging apparatus suffers from a few problems. For example, it generates a large amount of ozone, and in order to charge the image bearing member, it is necessary to apply high voltage, for example, 10 kV, between the charging apparatus and image bearing member, which adds to apparatus cost.
In recent years, the so-called contact charging apparatuses have been devised, and some of them have been put to practical use. In the case of this type of charging apparatus, the charging member of the charging apparatus is placed directly in contact with the image bearing member, and the image bearing member is uniformly charged by applying voltage to the charging member. In principle, however, this type of charging apparatus is the same as a corona type charging apparatus in that it also charges an object based on electrical discharge. Therefore, it also generates ozone, although by a smaller amount. Ozone forms nitric oxides (NOx), which are low in electrical resistance. Therefore, as nitric oxides adheres to the peripheral surface of the image bearing member, the image bearing member fails to be properly charged, resulting in the formation of defective images.
Thus, a charging process which does not suffer from the above described problem, that is, the ozone production, and is lower in the potential level of the voltage to be applied to a charging apparatus, has been proposed in Japanese Laid-open patent Application Hei 6-3921, or the like.
This charging process is characterized in that electrical charge is injected into the image bearing member through the direct exchange of electrical charge between the charging member, and the image bearing member surface placed in contact with the charging member, instead of electrical discharge.
Next, a charging apparatus for carrying out the above described charging process, or charge injection, will be described with reference to a sponge roller type charging apparatus (U.S. Pat. No. 6,128,456 or the like).
Referring to FIG. 5, the contact charging member of this type of charging apparatus is made up of a sponge roller 2-A, which is rotated in the direction b in contact with the image bearing member 1, and electrically conductive microscopic particles m (relatively low in electrical resistance) adhered to the peripheral surface of the porous portions, that is, the outer layer, of the sponge roller 2-A. Electrical charge is injected into the image bearing member 1 from the sponge roller 2-A at the contact area n, as the sponge roller 2-A is rotated in the direction counter to the rotational direction a of the image bearing member 1. As a result, the image bearing member 1 is charged to a potential level virtually identical to that of the electrical charge of the sponge roller 2-A.
The electrically conductive microscopic particles m are particles for enhancing the charging performance of the charging apparatus. As for the material for the electrically conductive microscopic particles m, various substances can be used; for example, microscopic particles of electrically conductive metallic oxide such as zinc oxide, microscopic particles of electrically conductive particles of inorganic substance other than metallic oxides, mixture of microscopic particles of electrically conductive inorganic and organic substances, and the like.
In this system, a DC voltage of xe2x88x92600 V is applied to the sponge roller 2-A from a power source S1. This voltage acts to raise the potential level of the portion of the image bearing member 1 in contact with the sponge roller 2-A and electrically conductive microscopic particles m to the same potential level as that of this voltage, that is, xe2x88x92600 V. If electrical charge from the sponge roller 2-A side can break through the barrier, or surface energy, of the peripheral surface of the image bearing member 1, it is injected into the image bearing member 1, charging the image bearing member 1. If electrical charge fails to break through this energetic barrier, the image bearing member 1 is not charged. Further, if electrical charge having been injected into the image bearing member 1 moves back from the image bearing member 1 to the sponge roller 2-A when the sponge roller 2-A is separated from the image bearing member 1, the image bearing member 1 does not remain charged. These phenomena are greatly affected by the energetic barrier of the peripheral surface of the image bearing member 1, and the charge retaining ability of the image bearing member 1. On the other hand, if a charging process is viewed as a process comprising a plurality of competing subordinate processes, the frequency at which the sponge roller 2-A makes contact with the image bearing member 1 is very important.
As for the means for increasing this frequency, it is effective to improve the state of contact between the sponge roller 2-A and image bearing member 1. The state of contact between the sponge roller 2-A and image bearing member 1 can be improved by adhering the electrically conductive microscopic particles m to the porous portion, or the surface layer, of the sponge roller 2-A, and/or by increasing the relative speed between the peripheral surfaces of the sponge roller 2-A and image bearing member 1 by making the moving direction of the peripheral surface of the sponge roller 2-A opposite to that of the peripheral surface of the image bearing member 1. With the provision of the above described arrangements, the peripheral surface of the image bearing member 1 is charged to a potential level virtually the same as that of the voltage applied to the sponge roller 2-A, that is, xe2x88x92600 V, uniformly, even in microscopic terms.
FIG. 6 is a schematic drawing of an example of an electrophotographic image forming apparatus which employs, as a means for charging the image bearing member 1, an injection type charging apparatus 2 which uses the above described electrically conductive microscopic particles m. This apparatus does not have a dedicated cleaning system, and employs a transfer type image formation system.
Designated by a referential code I is a rotational electrophotographic photoconductive member, in the form of a drum, which is rotationally driven at a predetermined peripheral velocity in the clockwise direction indicated by an arrow mark a. Designated by a referential code 2-A 2 is a sponge roller as a charging member, which is kept in contact with the image bearing member 1, with the application of a predetermined amount of pressure, forming a contact area n with a predetermined width in terms of the circumferential direction of the sponge roller 2. A referential code 2-B stands for a coating device for coating the peripheral surface of the sponge roller 2 with electrically conductive microscopic particles. As the sponge roller 2 is rotated in the clockwise direction indicated by an arrow mark b, the peripheral surface of the sponge charge roller 2 is coated with the electrically conductive microscopic particles m.
The peripheral surface of the image bearing member 1 is uniformly charged to predetermined polarity and potential level, as a predetermined charge bias is applied to the sponge charging roller 2 from the power source SI while the sponge charge roller 2 is rotationally driven in the direction counter to the rotational direction a of the image bearing member 1, with the electrically conductive microscopic particles m interposed in the contact area n, that is, the charging station, between the sponge charging roller 2 and image bearing member 1.
The uniformly charged peripheral surface of the image bearing member 1 is exposed by an unshown exposing means (digital scanning apparatus such as a laser beam scanner image projector for focusing the image of original, and the like)xe2x80x94a beam of light L reflecting image formation data is projected onto the uniformly charged peripheral surface of the image bearing member 1 from the exposing apparatus. As a result, an electrostatic latent image reflecting the exposure pattern is formed on the uniformly charged surface of the image bearing member 1.
Next, the electrostatic latent image is visualized as a developer image (toner image) by the sleeve 3-a of a noncontact (jumping) developing apparatus 3, in the development station f. Designated by a referential code t is the toner in the developing apparatus 3, and designated by a referential code c is the rotational direction of the development sleeve 3-a. Designated by a referential code S2 is a power source from which a predetermined development bias is applied to the development sleeve 3-a. 
Next, in the transfer station g, that is, the contact area between the a transfer roller 5-a of a transferring apparatus 5 and the image bearing member 1, the development image is transferred onto a transfer medium p as recording medium delivered, with a predetermined control timing, from an unshown sheet feeding station to the transfer station g. Designated by a referential code d is the rotational direction of a transfer roller 5-a, and designated by a referential code S3 is a power source from which a predetermined transfer bias is applied to the transferring apparatus 5.
After the reception of the developer image in the transfer contact area g, the transfer medium p is separated from the image bearing member 1, and is introduced into an unshown fixing apparatus, in which the developer image is fixed. Thereafter, the transfer medium p is discharged as a print or a copy.
After the separation of the transfer medium p, the residual developer particles, that is, the developer particles remaining on the peripheral surface of the image bearing member 1 after the image transfer, are carried by the rotation of the image bearing member I through the charge station, and then, the development station, in which the residual developer particles are removed from the peripheral surface of the image bearing member 1 by the developing apparatus 3 at the same time as the latent image formed on the peripheral surface of the image bearing member 1 is developed by the developing apparatus 3, during the following rotation of the image bearing member 1.
The polarity of the electrically conductive microscopic particles m is made opposite to that of the developer t. Therefore, the electrically conductive microscopic particles m are not transferred onto the transfer medium, remaining on the image bearing member 1, and then, are recovered (picked up) by the sponge charging roller 2-A; in other words, the peripheral surface of the image bearing member 1 is cleared of the electrically conductive microscopic particles m, being restored for charge injection. With the provision of the above described structural arrangement, even if the developer t accumulates on the sponge charging roller 2-A, as long as the electrically conductive microscopic particles m are supplied by an amount large enough to overwhelm the effects of the developer t having accumulated on the sponge charging roller 2-A, it is possible to prevent the image bearing member 1 from being unsatisfactorily charged.
In the case of this type of image forming apparatus, positive voltage is applied to transfer the image on the image bearing member 1. Therefore, after the image transfer, developer particles remaining on the image bearing member 1 will have been positively charged. These positively charged untransferred residual developer particles are given a proper amount of negative charge while passing between the electrically conductive microscopic particles m and image bearing member 1 while the image bearing member 1 is being charged. Thus, while they pass through the area in which the development process is carried out, they are recovered by the developing apparatus 3; they are not allowed to pass the development station. In other words, a cleaner-less electrophotographic process is carried out.
The electrically conductive microscopic particles m can be supplied to the contact area n, or the charging station, between the sponge charging roller 2-A and image bearing member 1 also from the developing apparatus 3 (U.S. Pat. No. 6,128,456). In this case, the electrically conductive microscopic particles m are mixed in advance with the developer particles, so that the electrically conductive microscopic particles m are adhered to the peripheral surface of the image bearing member 1 by the developing member, and are carried (supplied) to the contact area n as the charging station, by the rotation of the developing member.
The present invention is an improvement regarding an image forming apparatus in which electrically conductive microscopic particles are supplied to the contact area between the charging member and image bearing member from the developing means, by way of the image bearing member.
Thus, in order to reliably charge the image bearing member using the above described structure, it is mandatory for the electrically conductive microscopic particles m to be reliably supplied to the peripheral surface of the sponge charging roller 2-A. However, when the toner image formed of the developer t on the peripheral surface of the image bearing member 1 is transferred onto the transfer medium p by the transferring apparatus 5, all the developer particles in the toner image do not necessarily transfer onto the transfer medium p; some of them remain on the image bearing member 1 and reach the sponge charging roller 2-A, accumulating on the sponge charging roller 2-A. The accumulation of the developer particles t on the peripheral surface of the sponge charging roller 2-A adversely affects the charging performance of the sponge charging roller 2-A. Therefore, it is possible that as the developer particles t accumulate on the peripheral surface of the sponge charging roller 2-A as described above, the image bearing member 1 will fail to be properly charged.
On the other hand, during a paper interval, an image is not formed, and therefore, there is no developer particle t which moves from the image bearing member 1 to sponge charging roller 2-A. Thus, the electrically conductive microscopic particles m can be more reliably supplied onto the peripheral surface of the sponge charging roller 2-A during a paper interval than during an image formation period. If a paper interval is shorter than the time it takes for the sponge charging roller 2-A to make one complete rotation, it is impossible for the electrically conductive microscopic particles m to be supplied to the sponge charging roller 2-A across its entire range in terms of its circumferential direction, and therefore, it is possible that the charging performance of the sponge charging roller 2-A will become uneven in terms of its circumferential direction.
The primary object of the present invention is to provide an image forming apparatus in which electrically conductive particles are reliably supplied to the charging member.
Another object of the present invention is to provide an image forming apparatus in which electrically conductive particles are supplied to the image bearing member regardless of the pattern of the image formed on the image bearing member.
Another object of the present invention is to provide an image forming apparatus in which electrically conductive particles are uniformly supplied to the charging member.
Another object of the present invention is to provide an image forming apparatus in which electrically conductive particles are supplied from the developing device to the region of the image bearing member corresponding to a paper interval, by a large amount.
Another object of the present invention is to provide an image forming apparatus suitable for a cleaner-less system, that is, a system lacking a dedicated cleaner.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.